Talk:Perpetual motion machine

Perpetual motion machine

I'm sorry, but I think you're just wrong about the meaning of the phrase "perpetual motion" as used in discussions of thermodynamics, statements about the impossibility of perpetual motion, etc.

The perpetual motion, then, which has been the subject of such anxious and laborious search, is not a mere motion which is continued indefinitely. If it were, the diurnal and annual motion of the earth, and the corresponding motions of the other planets and satellites in the solar system, as well as the rotation of the sun upon its axis, would be all perpetual motions....

In short, a perpetual motion would be a watch or clock which would go for so long as its mechanism would endure without being wound up: it would be a mill which could grind corn or work machinery without the action upon it of water, wind, steam, animal power, or any other moving force external to it.

—Dionysis Lardner, 1857, Natural Philosophy for Schools, p. 70

Perpetual motion is of course possible, as is stated in the first law of motion, "Motion continues in a straight line undiminished in velocity unless acted on by some external force." A top set spinning under conditions where there is no friction will never cease to spin. These conditions are very nearly realized in the motions of the planets and stars. The statement that energy cannot be destroyed asserts this possibility.

But by "perpetual motion" is generally meant a machine which will do work and keep going though energy equivalent to the work done is not supplied to it. Many attempts have been made to construct such a machine. They have all failed. The statement that energy cannot be created denies the possibility of such a machine.

Dpsmith, I moved this discussion to the talk page for perpetual motion, so others can benefit and contribute as desired to this.

Your quotes are helpful, and I agree that there are different definitions for a perpetual motion machine. But everyone agrees that a machine that does work without energy is impossible. So the above definitions do not lead to productive discussion. Besides, the earth is not really a perpetual motion machine. Our orbit would eventually become unstable, and presumably we would slow down eventually.

What is a fascinating question is whether motion can continue indefinitely in a closed system. One quote above implies it can, but I doubt most would agree. I wouldn't. --Aschlafly 18:59, 1 January 2007 (EST)

He's right Mr. Schlafly; the impossibility of perpetual motion is proven by the 1st law of thermodynamics, not the second.

Ben

OK, Ben, but define perpetual motion machine in a trivial away and it only results in a trivial answer.

Now that's an interesting question that is difficult to answer fully! --Aschlafly 19:05, 1 January 2007 (EST)

Theoretically it is possible (the first law of motion), but practically it is impossible just because you wouldn't be able to eliminate all outside forces.

I also have a comment on your proof using the 2nd law of thermodynamics:

"The only argument that a perpetual motion machine is impossible is based on an interpretation of the Second Law of Thermodynamics. If entropy is always increasing, even in closed (and isolated) systems, then indefinite motion is impossible because an increase in the disorder of the system will inevitably disrupt the motion"

Firstly, as Dpbsmith has demonstrated, this is not the only argument against perpetual motion.

Secondly, though it is almost inevitable that increaced disorder will disrupt motion, it still isn't logically proven; it is always possible that it might go on just a little bit longer.

So, Ben, you seem to be saying that a "perpetual motion machine II" (as defined above) may be possible to build.

Really think so? If you doubt it, then maybe you can discovery a way to prove it is impossible. I don't think anyone has proven that yet, and no one else seems to be trying at this time either. But it would be worth proving.
--Aschlafly 23:53, 1 January 2007 (EST)

Aschlafly: you said "But everyone agrees that a machine that does work without energy is impossible." Well, no. Historically the whole debate arose because for centuries people have believed that they have found a way to build a machine that does work without energy, something for nothing. There are still people who believe it. They tend to use the terms "free energy" and "overunity" to avoid the onus of the term "perpetual motion." An example of a modern machine claimed by its inventor to generate more power than it consumes is the Adams Motor[1], [2]. A classic fraud was the Keely motor.

My point is that the phrase "perpetuum mobile" goes back to the days before thermodynamics and refers to innumerable attempts to produce simple mechanical arrangements, typically involving shifting or pivoting weights, that on paper look as if they might really do work without an external input.

You seem to be interested in a different philosophical question, one which I don't know much about or whether there is any established name for it. I'm arguing strongly that you should either find out what that name might be, or invent something that doesn't use the phrase "perpetual motion" at all.

Here is my guess at what an answer to your question might be. If you use the best modern techniques, e.g. suspending a spinning object in vacuum via superconductive magnetism, you can get something that will move without additional energy input for a really long time. You can measure the rate at which it slows down very carefully or predict it theoretically.

My guess is that it may well be possible today to build a system in which the rate of energy loss is so slow that it can be predicted continue to run for hundreds of thousands of years if the apparatus remains intact; thus the limiting factor in how long it runs is not the apparatus itself, but unrelated external catastrophes (an asteroid hits it, the building it is in collapses in an earthquake, funding runs out and someone pitches the apparatus in the trash, breaking it, etc. etc.)

Probably the place to look for one of these things in real life would be the gyroscopes used in inertial guidance systems.

If the word "forever" is taken literally, then I don't know how you answer the question, because beliefs about the future lifetime of the universe change every generation or so. If the word "forever" means something like a mathematical limit—then I think the answer is: according to current understanding, it is possible to build a machine that will run indefinitely long, or as long as you like, receiving no external energy and performing no external work, where the actual limit on the running time is set not by the construction of the machine itself, but by the probability of external catastrophe.

OK, Dpsmith, you win on your point about what a perpetual motion machine really means. But I'm still interested in why perpetual motion (without producing extra energy) is impossible. The increase in entropy must prevent it. Underlying that may be the uncertainty principle in quantum mechanics.

More thought and research would be worthwhile here. I think we're all convinced that the motion would eventually stop. But why? What force stops it? --Aschlafly 22:36, 2 January 2007 (EST)

It's beyond my own knowledge. I suspect such questions are like the irresistable force and the immovable object, though. Provisionally, let's call your gadget an "endless coaster."

Point #1: At least according to Newtonian physics as I understand it, if you truly had a closed system, it would not stop. But if you truly had a closed system, there would be no way to see that it was still moving.

In order to observe it, there would have to be some energy exchange between the "perpetual motion" and the observer. I have an idea that since the observer is gaining information, the observed must be gaining entropy, but that's just handwaving and I don't know how to prove it.

What I don't know whether there are any theoretical reasons that would make it impossible to have a truly closed system.

Here's another angle. If we're considering, say, a ball bearing magnetically suspended in a vacuum by a superconducing magnet or something like that, if if there is a small amount of friction the result, according to classical physics, would be to make the ball bearing spin slower on an exponential decay curve. It would have a half-life, like radioactive decay. Perhaps it loses half of its spin every day. Well, according to classical physics, it would spin slower and slower but would never actually stop. Asymptotically approaches zero, never eaches it. Most likely (out of my depth again) quantum physics would say that at some point the spin becomes quantized, meaning that after some period of time it can't slow down any more. The spin must be either one quantum or none... and then you get all that crazy wave-function collapse stuff. You have a superimposed state in which the spin is one with some probability and zero with some probability, and the probability decreases over time.

But really, once you start talking about whether something can literally go on forever, you're outside the bounds of science. I can't keep track of the number of times the "scientific" narrative of cosmology has changed during my own lifetime, and it shows no signs of settling down. Is the universe closed? Open? Continuously expanding? Oscillating? Obviously, if physics predicts a finite lifetime for the universe, then a "perpetual" motion, meaning one that would last longer than the universe, is impossible. Dpbsmith 10:03, 3 January 2007 (EST)

That's an interesting point of linking observation to entropy. But I do think even a purely closed system would stop without observation. Don't you? Perhaps Newton would not be pleased, but the Second Law of Thermodynamics suggests that motion does eventually stop. --Aschlafly 00:57, 5 January 2007 (EST)

No, I don't think it does. The question here is whether anything says how fast entropy increases... and what counts as "motion."

I think that the Second Law applies to large systems with many interacting particles or bodies and is some kind of statement about their statistical behavior and how easily that motion can be observed.

Imagine, say, an ideal, large, sealed box whose walls perfectly hard (do not flex or absorb energy), and imagine that it contains one ideal moving billiard ball. By an "ideal billiard ball" I mean, again, one that is perfectly hard and perfectly elastic. If you have a single billiard ball in the box and it is moving, I think it keeps bouncing off the walls and moves forever. After all, energy is conserved.

Now, suppose, instead, that you have twenty-one ideal billiard balls, twenty of them at the vertices of an icosahedron and one in the center, all connected to each other by ideal springs. The entire structure, which I'll call a "blob," resembles a non-ideal ball. Put one of these into the ideal box and set it in motion with a gentle and identical force on each of those billiard balls, so that they are not moving with respect to each other and the whole blob moves together.

Initially, the blob moves as a whole, and you can calculate the kinetic energy just by observing the blob; 1/2 mv2 where m is the total mass of the blob and v is the velocity of the blob as a whole.

But when it strikes the walls, the billiard balls are going to hit it at more or less random times. The result is that the balls in the blob are going to start to acquire motion relative to each other, and soon there is going to be lots of relative motion within the blob.

This relative motion represents kinetic energy that belongs to individual billard balls within the blob, not to the blob as a whole, so because of conservation of energy, the energy we can ascribe to the blob as a whole is going to decrease, and so is the average velocity of the blob.

I think that what the Second Law is saying is that the way in which the blob hits the wall is essentially random, and that with each impact, statistically, more and more energy is going to end up in the form of billard balls oscillating with respect to each other within the blob, and less and less in the form of organized motion of the entire blob as a whole.

So that whereas the motion of the single billard ball "never stops," after a while the motion of the blob has stopped, and instead you just have a stationary blob with the billard balls within it oscillating on their springs.

In other words, the behavior of the system has degraded from observable motion of the blob as a whole to less-observable relative motion of the billiard balls within the blob. The system is in a less organized or "heat-like" state.

However, because in this case we're talking about fairly large particles and a fairly small number of them, it is clear that the system is still "in motion," just on a smaller scale, and since we posited that the box, the springs, and the billiard balls are all ideal (and don't absorb energy), by conservation of energy the balls within the blob also continue in motion forever.

Now, we go one step further and still keep the idealized, closed system with vacuum and perfect walls, but instead of a billard ball we use a real rubber ball. What the Second Law says is that the mechanical energy of the bouncing ball, 1/2 mv2 where we can measure the "velocity" of the ball as a whole, inevitably and statistically degrades into heat; the ball "loses energy" with each impact with the wall, the measurable v decreases, and eventually it comes as close to "stopping" as we like. Conservation of energy says energy hasn't really been lost; it's been transformed into heat energy. The ball is warmer than before, meaning the molecules within it are moving, and since we've defined the system to be closed, it won't cool down. It has stopped moving, but there is still motion.

So, I think the whole thing becomes a sterile exercise in what we mean by "forever," and how close we can approximate ideal conditions with realizable machinery, and whether the motion of molecules due to heat counts as "motion."

I don't think Second Law has anything to say about how fast entropy increases, or how close we can come to an ideal situation where entropy doesn't decrease at all.

In a way the two are related, because "frictionless pivot," for example, means "no entropy increase in the form of heating at the pivot."

Probably the place where the Second Law comes into play is that it says that even if you have a perfectly idealized "closed system," within that system energy won't be lost, but nevertheless energy observable as macroscopic motion can still degrade into heat energy no longer observable as macroscopic motion.Dpbsmith 09:40, 5 January 2007 (EST)

The Uncertainty principle doesn't invalidate Perpetual motion...

The Heisenburg Uncertainty Principle applies to sub-atomic particles like electrons only, we cannot know where they are at any given time because of Brownian motion. It does not apply to anything that can be seen without the aid of an electron microscope. Also, entropy doesn't increase, and the Second Law of Thermodynamics merely states that the universe tends towards Entropy. Also, the lack of creation of energy would be the Law of Conservation of Energy, which is present in many parts of Physics beyond the First Law of Thermodynamics. Also, the Earth is not a perpetual motion machine, to argue that it is does not border, plunges headfirst into absurdity and ignorance. Finally, the Earth has been slowing at a rate of about 2.2 seconds every 100,000 years due to frictions, no one considers it a perpetual motion machine. JanSmuts 16:26, 12 April 2012 (EDT)

No, Brownian motion is associated with sub-atomic particles, but has to do with collisions at an observable level, rather than their placement in shells. It would appear we are both incorrect, my mistake. JanSmuts 17:02, 12 April 2012 (EDT)

I'm sorry, but you remain totally confused about Brownian motion even after acknowledging your earlier error. Brownian motion was first described by the biologist Robert Brown around 1827. He was describing a microscopic phenomenon and not a molecular one and certainly not a sub-atomic one. If your understanding of basic terms is so faulty I am not surprised that your conclusions are bad.--DavidEdwards 10:11, 13 April 2012 (EDT)

Another appeal to hearsay? The question doesn't request more hearsay.--Andy Schlafly 18:38, 12 April 2012 (EDT)

Sorry, you'll have to explain that comment. --JeromeKJ 18:48, 12 April 2012 (EDT)

According to all known laws of physics, no, such a machine cannot exist. However, the proofs you have offered are quite misleading and examples of bad science to say the least. JanSmuts 23:04, 12 April 2012 (EDT)

"no, such a machine cannot exist." Why? And in response to the prior comment above, I'm not asking what is "usually the answer" by others (i.e., hearsay). I'm asking for your opinion and explanation.--Andy Schlafly 23:49, 12 April 2012 (EDT)

Can I please point out that the 2nd Law, at present, does NOT have any consistent and/or accepted deep physical mechanism behind it involving quantum mechanics or whatever. It can be justified by the argument that systems will naturally move towards larger homogenous volumes in phase space, but the question of WHY we think the 2nd law is valid is still entirely open. The Uncertainty Principle as normally presented (i.e. the product of uncertainties between certain pairs of eigenvalues of observables - specifically, those that don't commute with each other) in no way has any time-asymmetry in it. The 2nd law DOES have time-asymmetry. There's no argument that you, I, or any of the most learned physicists in the world can currently make which directly attributes "entropy" to "quantum uncertainty". If you have one, with the mathematics to back it up, please submit it to Nature, and prepare for a trip to Stockholm... Dan W

I agree that narrow interpretations of the Second Law are, well, limited in what they conclude. But a full view of the meaning of the Second Law does recognize that quantum uncertainty underlies it. Quantum uncertainty does have a time-asymmetry to it ... just as attempts at a perpetual motion machine do.--Andy Schlafly 22:33, 5 May 2012 (EDT)

Interesting; as I said before, if you indeed can demonstrate this, and were subsequently denied a Nobel prize for it, I may accept your opinion that the committee is biased against those with a conservative viewpoint. Please point me to a derivation / demonstration of the uncertainty principle (other than those I have come across in my career, which are based on the Fourier transform of a wavepacket, for example, and which are not inherently time asymmetric) to back up your claim. Alternatively, please point me towards evidence that all current holders of the Nobel prize are liberals. User:DanPW

As to the time asymmetry of quantum mechanics, the uncertainty yields greater uncertainty over time, as in quantum tunneling.--Andy Schlafly 17:47, 6 May 2012 (EDT)

As to your "argument" about quantum tunnelling, I see that since you clearly have absolutely no understanding of what these concepts actually mean, there's little point in arguing this anymore. For the record, though, quantum tunneling arises due to the non-zero amplitude of a wavefunction in a classically forbidden region of a potential. The uncertainty principle is not what "causes" tunneling, rather both tunneling and the uncertainty principle are consequences of the wave nature of the statefunction. As to the Nobel prize, 1) whether or not you agree with Obama winning it (I certainly do not think he should have!), the Nobel Peace prize should be separated from the scientific prizes since it's clearly actually something to do with politics, wheras the other prizes are not, save perhaps for economics. Also, the fact that the committee has sometimes missed out on giving deserved awards (e.g. Franklin, Burnell, Hoyle, not so convinced about Dicke but I do recognise he was a great scientist, particularly in the area of electronics), does not give you any evidence of a liberal bias. As far as I can see, no mention has ever been made to the politics of any of these people. Fred Hoyle might well have been a liberal, or a conservative, we have no idea. User: DanPW